Integrated microtoroids monolithically coupled with integrated waveguides

ABSTRACT

A photonic apparatus comprises an integrated waveguide, an integrated resonator in the form of a microtoroid and a thermally reflowable film. The reflowable film comprises a first film area and a second film area. The reflowable film is one of a thin film and a stack of thin films. The first film area is thermally reflown, the microtoroid is formed in the thermally reflown first film area. The second film area is not reflown in the immediate vicinity of the microtoroid. The microtoroid is optically coupled to the integrated waveguide located on or located within one of or both of the first or second film areas. The first and second film areas are directly connected to each other. The microtoroid has an edge extending along a circumference. The microtoroid can be a non-inverted or an inverted microtoroid, wherein the second film area is inside or outside of the circumference.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Phase of International ApplicationNo. PCT/EP2011/067934, filed Oct. 13, 2011, which, in turn, claims thebenefit of U.S. Provisional Patent Application No. 61/393,170, filedOct. 14, 2010, and U.S. Provisional Patent Application No. 61/437,668,filed Jan. 30, 2011, the contents of which are hereby incorporated byreference in their entirety as part of the present disclosure.

FIELD OF THE INVENTION

The invention relates to a photonic apparatus, a method for fabricatinga photonic apparatus and a microtoroid. The invention consists in anovel type of integrated resonator that can be monolithically coupled tointegrated waveguides while allowing at the same time extremely highquality factors.

BACKGROUND OF THE INVENTION

Microtoroids are a form of integrated resonator that can have extremelyhigh quality factors on the order of 100e6. The geometry and fabricationof microtoroids is taught in “Ultra-high-Q toroid microcavity on a chip”by D. K. Armani, T. J. Kippenberg, S. M. Spillane, K. K. Vahala, Nature421, 925-928 (2003). In particular, microtoroids can be formed bythermally reflowing a material, so that the surface of the microtoroidcan be shaped by surface tension and can be extremely smooth. This isthe mechanism that allows such high quality factors, since scatteringlosses due to surface roughness are almost completely removed. Highquality factors are very important for a number of applications such asoptical time delay lines, narrow optical notch filters, or lightgeneration in resonators via non-linear processes.

Microtoroids are a form of whispering gallery resonator in that they areessentially a waveguide forming a closed loop in which light cancirculate. Due to the manner in which microtoroids are fabricated, theyare also substantially planar, in that the height of the loopedwaveguide is constant relative to the chip surface and determined by theinitial position of a thin film out of which the microtoroid isfabricated via a reflow process. In order to distinguish the waveguideforming the microtoroid from the waveguide to which the microtoroid iscoupled to in this invention, the former is explicitly referred to asthe looped waveguide in the following, while the latter is referred toas the integrated waveguide or coupled to waveguide.

An important drawback of these microtoroids is that they are extremelydifficult to couple to integrated waveguides that are fabricated in thesame chip than the microtoroids, i.e. to monolithically integrate themwith waveguides. This is due to the fact that there is typically nomechanical connectivity between the portion of the thin film out ofwhich the microtoroids are made by locally thermally reflowing said thinfilm and the remaining portions of said thin film elsewhere on the chip.For this reason, there is no mechanically supportive layer on which orin which a coupled to waveguide can be fabricated without this waveguidebeing trapped within the circumference of the microtoroid. The thin filmout of which microtoroids are made by locally reflowing said thin filmis referred to as the reflow film in the following. Monolithicfabrication of both the microtoroid and of the coupled to integratedwaveguide would allow ultra precise positioning of the coupled towaveguide relative to the microtoroid, with the accuracy of lithographicfeature definition, and would allow removing the high assembly costsincurred when assembling a microtoroid with a discrete fiber orwaveguide, i.e. with a coupled to device that is not fabricated on thesame chip.

A typical microtoroid is fabricated by starting with a thin-film out ofa material that can be thermally reflown, such as a silicon dioxidefilm, on top of another material, such as for example a siliconsubstrate. A disk is first etched into the silicon dioxide thin film.The disk is than undercut by partially removing the silicon substratefrom below the disk. This partial removal of the silicon is typicallyachieved with an isotropic etch such as is achieved by etching with gasphase XeF₂. This results in a suspended disk located on top of apedestal and attached to the rest of the chip via said pedestal. The rimof the disk is then locally reflown so that it forms a microtoroid. Thereflow process is typically induced by heating with a C0₂ laser with alaser wavelength of 10.6 μm. The absorption coefficient of silicondioxide is much higher than the absorption coefficient of silicon at10.6 μm, so that the silicon dioxide film can be selectively heatedrelative to the silicon. Furthermore, the laser spot can be focused ontothe disk, so that the silicon dioxide film can be locally heated in thevicinity of the disk. Finally, the silicon pedestal acts as a heat sink,so that the silicon dioxide heats up most at the rim of the disk, wherethe distance to the pedestal is the largest and the heat sinking theleast efficient. This way, with the correct combination of C0₂ laserpower, exposure time and focusing, the silicon dioxide film can bemolten around the rim of the disk while remaining unmolten at thecenter. The silicon dioxide reflows at the rim forming an extremelysmooth microtoroid with a circumference defined by the initial rim. Dueto the reflow process, the looped waveguide is thicker than the reflowfilm out of which it is fabricated, thus forming a structure in whichlight can be guided.

Light is typically coupled to such a microtoroid from a tapered fiber,and the coupling coefficient between the fiber and the microtoroid tunedby adjusting the distance between the tapered fiber and the microtoroid.The tapered fiber is a suspended structure in the vicinity of themicrotoroid in that it is surrounded by air.

Some attempts have been made to couple an integrated microtoroid with anon-chip waveguide. Such a structure is taught in “Silicon MicrotoroidalResonators with Integrated MEMS Tunable Coupler” by Jin Yao, DavidLeuenberger Ming-Chang M. Lee, and Ming C. Wu, IEEE Journal of SelectedTopics in Quantum Electronics, Vol. 13 (2007). In this structure afreestanding Silicon Waveguide is coupled to a non-inverted Siliconmicrotoroid resonator. This structure is fragile due to thefree-standing waveguide, is complicated to fabricated, and it remainsunclear whether it can be applied to microtoroids made out of silicondioxide for which much higher quality factors can be achieved than forsilicon microtoroids. It is essentially identical to the one describedin the previous paragraphs in that the microtoroid is made by reflowinga disk and in that the coupled to waveguiding element is suspended inair and held in close proximity to the microtoroid.

Other shapes of microtoroids can be fabricated with the above-describedmethod by etching other shapes into the silicon dioxide film, such asracetracks, ovals etc. This results in microtoroid type resonators forwhich a cross-section, typically a substantially circular cross-section,forms a closed loop mechanically connected to a non-reflown portion ofthe silicon dioxide film 6 and attached to the rest of the chip via apedestal connected to said non-reflown portion of the silicon dioxidefilm. This non-reflown portion connected to the pedestal is inside thecircumference of the microtoroid.

SUMMARY OF THE INVENTION

Instead of first etching a disk or other shape such that the remainingreflow film (e.g. silicon dioxide film) is inside the contour of theshape (inside the rim), a hole for which the remaining reflow film isoutside of the contour of the shape (outside of the rim) is etched intothe reflow film and used as a starting feature (precursor) to define themicrotoroid. The remaining reflow film comprises a first film area and asecond film area. The remaining reflow film is undercut by partiallyetching away the substrate. The edge (rim) of the hole and the firstfilm area are then locally reflown. This results in a microtoroid whosecircumference is also defined by the rim and that is directly attachedto the second film area of the reflow film, i.e. the non-reflown portionof the reflow film via the outer edge of the microtoroid. Saidnon-reflown portion of the reflow film is connected to the rest of thechip via a section of the substrate material located below the reflowfilm. The non-reflown portion of the reflow film connected to themicrotoroid and to the rest of the chip via the section of substratematerial is located outside of the circumference of the microtoroid, andnot inside of the circumference. Here too, the microtoroid can take avariety of shapes such as a racetrack, an oval etc. provided the rim ofthe hole is defined accordingly. This structure is further referred toas an inverted microtoroid.

This results in a structure in which an integrated waveguide can bedefined inside the reflow film or on the reflow film in such a way thatthe waveguide is coupled to the microtoroid, the waveguide is locatedoutside of the circumference of the microtoroid and can be routed toother locations on the chip outside of the circumference of themicrotoroid without having to cross the microtoroid, and the waveguideis mechanically supported by the thin film out of which the microtoroidis made by partially reflowing said thin film (the reflow film), so thatit does not need to be free standing (suspended) in order to be in closeproximity to the microtoroid as is required in order to obtainsubstantial coupling with the microtoroid. In particular, the integratedwaveguide can be in direct mechanical contact with the reflow film in anentire coupling region or in the entire chip.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section of a non-inverted microtoroid with a coupledto integrated waveguide.

FIG. 2 is a cross-section of an inverted microtoroid coupled to anintegrated waveguide.

FIG. 3 is a cut through of an non-inverted microtoroid without a coupledto integrated waveguide located on or inside of the reflow film, such asis known in the prior art, is coupled to a tapered fiber suspended inclose proximity to the microtoroid, as is also known in the prior art.

FIG. 4 is a 3D rendering of an inverted microtoroid coupled to anintegrated waveguide 4 located on or within the reflow film.

FIG. 5 is a graphical representation of an inverted microtoroid having athinner reflow film geometry (left) and an inverted microtoroid having athicker reflow film geometry (right), showing the respective opticalmodes with low bending losses.

FIG. 6 is a graphical representation of an inverted microtoroid havingan off center microtoroid loop waveguide relative to a middle horizontalplane of the reflow film.

FIG. 7 is a representation of several possible fabrication flows withsuccessive device cross-sections.

FIG. 8 is a representation of a microtoroid coupled to an integratedwaveguide with a Silicon Nitride core.

FIG. 9 is a representation of a microtoroid coupled to an integratedwaveguide with a box shaped Silicon Nitride core.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The coupled to integrated waveguide core can be made out of a variety ofmaterials provided the refractive index of said materials is at leastpartially larger than the refractive index of the reflow film. This isrequired due to the fact that the waveguide core is embedded in thereflow film, or located on the reflow film, so that a higher refractiveindex is required in order to confine the light within the waveguidecore. In the case where the reflow film is silicon dioxide, such amaterial can for example be silicon, silicon nitride, or siliconoxi-nitride. The integrated waveguide cross-section can take a varietyof shapes, such as a rectangle, a partially etched thin film, or a boxshaped cross-section.

In the case of an integrated, coupled to silicon waveguide, a typicalfabrication flow consists in starting with silicon-on-insulator (SOI)material, fabricating the waveguide in the top silicon layer 23 of theSOI material, etching the hole whose rim defines the circumference ofthe microtoroid (the microtoroid precursor hole) such that the holereaches through the buried oxide (BOX) of the SOI material and down tothe bulk silicon (also called silicon handle) acting as the substrate ofthe SOI material, etching the substrate of the SOI material where it isexposed through the hole so as to undercut the BOX oxide and locallyreflowing the BOX oxide around the edges of the hole so as to fabricatethe microtoroid. This results in an apparatus in which the coupled towaveguide is located on top of the reflow film.

In the case of a silicon waveguide, another typical fabrication flowconsists in starting with silicon-on-insulator (SOI) material,fabricating the waveguide in the top silicon layer of the SOI material,depositing additional dielectric layers (the back-end dielectrics) suchas silicon dioxide or nitride poor silicon oxi-nitride or otherdielectrics whose reflow temperature is such that it can be reflown bythe applied reflow process and whose refractive index is lower than thewaveguide core, etching the hole whose rim defines the circumference ofthe microtoroid (the microtoroid precursor hole) such that the holereaches through the back-end dielectrics and the buried oxide (BOX) ofthe SOI material and reaches down to the bulk silicon (also calledsilicon handle) acting as the substrate of the SOI material, etching thesubstrate of the SOI material where it is exposed through the hole so asto undercut the BOX oxide and locally reflowing the BOX oxide around theedges of the hole so as to fabricate the microtoroid. This results in anapparatus in which the coupled to waveguide is located within the reflowfilm and is embedded inside the reflow film.

Optionally, the stack of back-end dielectric layers can also be locallythinned in the region where the microtoroid is later fabricated by localreflow. This is for example useful if the total back-end thickness istoo thick for the targeted microtoroid geometry, or if some of the topback-end dielectric layers cannot be reflown and need to be removed inthe vicinity of the rim of the hole. This can for example be implementedby an etch prior to the etch defining the microtoroid precursor hole andapplied to a region larger than and encompassing the precursor hole.This can also be implemented by an etch applied after the etch definingthe microtoroid precursor hole and applied to an area larger than andencompassing the precursor hole. This local thinning of the back-end canalso be useful if the back-end contains non-dieletric materials such asmetal interconnect layers that need to be removed in the vicinity of themicrotoroid circumference prior to fabricating the microtoroid.

Microtoroids can also be coupled to with other types of integratedwaveguides, such as waveguides with a silicon nitride core. Here too,the waveguides are typically fabricated first on top of the reflow filmor within the reflow film (in which case part of the reflow film isdeposited on top of the waveguides), optionally the back-end stack islocally thinned, and the microtoroid precursor hole etched (notnecessarily in that order, the hole can also be etched before theback-end is locally thinned), the exposed silicon substrate etched andthe BOX oxide undercut at the rim of the hole. Finally the microtoroidis fabricated by local reflow.

Such microtoroids can also be integrated with complex integrated optics,electronics and other on-chip devices. In a preferred fabrication flowthe complex integrated optics, including the coupled to waveguide, andthe other on-chip devices, including electronic devices such astransistors, are first fabricated, typically such that at the end ofthis first fabrication flow the back-end does not contain any metals inthe region where the microtoroid will later be defined, in particularthe region that will be subject to reflow. The back-end stack is thenoptionally thinned and the microtoroid precursor hole is etched (notnecessarily in that order, the hole can also be etched before theback-end is locally thinned), the exposed silicon substrate etchedthrough the hole and the BOX oxide undercut at the rim of the hole.Finally the microtoroid is fabricated by local reflow. In anotherpreferred fabrication flow metal layers are present in the back-end inthe region later submitted to reflow, but are etched away as part of theback-end thinning prior to reflow.

Electro-optic devices such as modulators or photodetectors, orelectronic devices such as transistors, are typically very sensitive totemperature excursions during fabrication, wherein such temperatureexcursions can permanently damage these devices if they exceed thethermal budget of the devices. This can be caused for example due tosilicide deterioration or due to implant diffusion. In the fabricationflows described here these thermal budgets are not additionallyburdened, or not excessively burdened, since the reflow film is onlyselectively heated due to a combination of local focusing of the C0₂beam and of local heat sinking. The reflow process can take other formsthan laser induced heating. For example, the chip can be heated by rapidthermal annealing (RTA) in which case the chip is also heated viaexposure to intense radiation. In this case the light is applied to theentirety of the chip, but here too the reflow film such as silicondioxide is heated faster than the substrate material and the local heatsinking mechanism remains in that most of the reflow film is heat sunkby the underlying substrate and only the undercut regions of the reflowfilm are not efficiently heat sunk. This way, selective reflow of thereflow film is maintained and the thermal budget of other devices can bemaintained. The reflow process can be done with other laser sourcesemitting a wavelength for which the absorption coefficient in the reflowfilm is higher than the absorption coefficient of the substrate, or withother radiation sources such as a RTA such that the overall absorptioncoefficient in the reflow film is higher than the overall absorptioncoefficient in the substrate (defined as the weighted average of theabsorption coefficients over the spectral density of the radiationsource).

After selective reflow of the reflow film and definition of themicrotoroid, the microtoroid can be embedded in an additional materialsuch as a transparent epoxy, a non-linear polymer (for example withsecond order or third order optical non-linearities), or anothermicrotoroid cladding material. Such layers are typically at leastpartially organic and spin coated onto the chip. Such materials can alsobe used to implement a bio-detector or a chemical detector if theiroptical properties such as index of refraction or optical absorption aredependent on the presence of a detected species. Such microtoroidcladding materials can also be a liquid that can be flown over themicrotoroid.

These fabrication flows also allow fabricating a non-invertedmicrotoroid with a disk precursor, or other precursor such that thenon-reflown portion of the reflow film connected to the microtoroid islocated within the circumference of the microtoroid, but such that themicrotoroid is monolithically coupled to a waveguide located within oron the reflow film.

The waveguide would then be confined to the region located within thecircumference. Since such a microtoroid typically has dimensions of 10to a few 100 μm across (for example the diameter of the disk) and atmost a few mm across, this leaves very little space to implement therest of the on-chip optical system and to couple to and from thison-chip optical system. Permanent attachment of off-chip opticalelements such as a fiber array or a planar lightwave circuit coupled tothe on-chip optical circuit typically requires large areas, larger thanthe area within the circumference of the microtoroid. Unless themicrotoroid is embedded in an additional cladding material, this isdifficult to achieve without touching the microtoroid, with the risk ofdestroying the mechanical integrity of the microtoroid, or at least itsoptical properties due to the close proximity of the other off chipelements. For this reason, the inverted microtoroid geometry for whichthe non-reflown portion of the reflow film to which the microtoroid isattached is located outside of the circumference of the microtoroid ispreferred when coupling a waveguide to the microtoroid. In such a casecoupled to waveguides can be routed far enough from the invertedmicrotoroid to allow attachment of large off-chip optical elementswithout touching or otherwise impacting the microtoroid. It also allowsthe rest of the optical system to fill up to the entire area of the chipoutside of the circumference of the microtoroid.

One possibility to protect the mechanical and optical properties of themicrotoroid is to apply a material to the chip such that it covers themicrotoroid, apply a curing step such that said material becomes solidor at least highly viscous such as thermal curing, ultra-violet curingor chemical curing and only attach off-chip elements that couldotherwise damage or impact the microtoroid after the curing step. Thiscorresponds to encapsulating the microtoroid in order to protect it fromfurther assembly steps.

Another possibility to protect the mechanical and optical properties ofthe microtoroid is to apply a material to the chip such that it coversthe microtoroid, to hold further off-chip elements at a sufficientdistance from the microtoroid so as not to compromise the mechanical oroptical properties of the microtoroid, and to cure the material so as toattach and hold the off-chip elements at a sufficient distance.

One of the difficulties in designing and fabricating the inversemicrotoroid structure is that light can be poorly confined to themicrotoroid, leading to high radiative bending losses and to low qualityfactors if the structure is not carefully designed and fabricated.Bending losses and radiative losses are used interchangeably in thisinvention description.

In a bent region of the looped waveguide forming the microtoroid, theoptical field is concentrated in the region of the bent loop waveguidetowards the outer region of the bend and partially radiates out of theloop waveguide. In the inverted microtoroid geometry, the microtoroid istypically attached to the reflow film at the outer edge of themicrotoroid and thus the outer region of the bend. This is for examplealways the case if the circumference of the microtoroid forms a convexshape such as a circle or a racetrack. The proximity of the reflow filmat the outer edge of the loop waveguide bend typically worsens radiativelosses, since light can couple into the reflow film that has a higherindex than the rest of the material surrounding the microtoroid.

In general, a thinner reflow film in the non reflown portion, a thickerlooped waveguide forming the microtoroid (i.e. with a largercross-section), and a larger outer microtoroid bending radius lead toreduced bending losses. Acceptable bending losses can be achieved. Forexample, an inversed microtoroid with a reflow film thickness of 1 μm, abending radius of 120 μm and a microtoroid inner radius of 3.7 μm leadsto a radiation losses limited quality factor larger than 100e6, at whichpoint one can assume that quality factors will be limited by materialproperties such as optical absorption through the material of the reflowfilm and residual scattering. This quality factor was calculated for theground mode of the microtoroid loop waveguide, i.e., the mode for whichthe loop waveguide has the highest effective index and that has a singlelobe mode profile.

For thick reflow film geometries the radiative losses of the ground modeof the loop waveguide can become excessive, but higher order loopwaveguide modes can maintain sufficiently low radiative losses to resultin quality factors in excess of 100e6. For example an inversedmicrotoroid with a reflow film thickness of 1.5 μm, a bending radius of120 μm and a microtoroid inner radius of 5.6 μm maintains a radiationlosses limited quality factor in excess of 100e6 for a higher order loopwaveguide mode that has two lobes in the vertical direction(perpendicular to the chip surface), while the quality factor of theground mode is much lower. This is due to the fact that the higher ordermode has an odd symmetry in the vertical dimension and cannot couple tothe ground mode of the reflow film that has an even symmetry. Thisassumes that the symmetry plane of the reflow film is not substantiallybroken by the loop waveguide.

The radiative losses for the higher order loop waveguide mode are lowerin the previous case due to the opposite symmetry of the higher orderloop waveguide mode relative to the ground mode of the non-reflownreflow film. This assumed a microtoroid loop waveguide that maintainsthe symmetry of the reflow film, i.e., that is symmetric relative to ahorizontal plane passing through the middle of the reflow film. Howeverthis symmetry can be broken by the reflow process in that themicrotoroid loop waveguide is typically off center with a loop waveguidecenter point that is higher than the middle plane of the reflow film(where higher refers to further above the surface of the chip). In anextreme geometry the microtoroid is pulled high enough such that thereflow film is connected to the lower edge of the loop waveguide. Evenin such an extreme geometry it is possible to maintain a radiativequality factor larger than 100e6. This is for example the case for theground mode of a microtoroid with a bending radius of 200 μm, an innerradius of 2.75 μm, and a reflow film thickness of 1.1 μm.

Due to the fact that high bending losses can occur due to the attachmentof the reflow film at the outer edge of the bend when the dimensions ofthe inversed microtoroid are not chosen carefully, it is not intuitiveto one skilled in the art that such a structure can be a high qualityfactor optical resonator.

Another difficulty consists in implementing the integrated waveguidethat is coupled to the microtoroid and is located on or within thereflow film. This is due to the fact that there are typicallyconflicting requirements on the reflow film thickness from the point ofview of maintaining a high quality factor resonance in the microtoroidand fabricating a coupled to integrated waveguide with low substratelosses. These conflicting requirements do not occur when the substratehas a refractive index that is substantially lower than the core of theintegrated waveguide. However, the substrate is typically silicon andthe waveguide core made out of silicon or out of a material with a lowerrefractive index than silicon, so that waveguide losses due to lightcoupling from the waveguide to the substrate can be an issue. Since thewaveguide is spatially separated from the substrate by the portion ofthe reflow film located below the waveguide, a thick reflow film isdesirable in order to reduce substrate coupling losses.

In the asymmetrical microtoroid loop waveguide geometry described above,a silicon waveguide can for example be embedded such that it is coveredby 200 nm of the reflow film as a top cladding layer, such that thewaveguide is 220 nm thick in the vertical direction and such that 680 nmof reflow film is located below the waveguide as a bottom cladding layerseparating the waveguide from the substrate. This results in a totalreflow film thickness of 1.1 μm compatible with previously describedgeometries. For this geometry the substrate coupling losses for a 500 nmwide waveguide are for example on the order of 2 dB/cm and remainacceptable. On places of the chip where straight waveguides are simplyrouting light, they can be made wide and multi-mode to maximize theconfinement of light inside the core area and minimize substrate losses.On the other hand in the coupling region to the microtoroid, thewaveguide is typically tapered to a much thinner cross-section, but overa sufficiently short distance for substrate coupling losses to remainacceptable. The integrated waveguide in the coupling region is alsotypically within or on a region of the reflow film that is undercut,further reducing substrate coupling losses.

A further difficulty resides in phase velocity mismatch between themicrotoroid loop waveguide and the coupled to integrated waveguide.Phase velocity matching is required in order to be able to obtain highcoupling between the integrated waveguide and the microtoroid loopwaveguide. Since the microtoroids typically have very high qualityfactors, very small coupling coefficients can be sufficient even inorder to obtain critical coupling, a coupling strength such that all thepower from the waveguide is dropped into the microtoroid at resonance.However, there are limits on the allowable phase velocity mismatch inorder to achieve targeted coupling coefficients. In particular, a goodphase velocity matching allows the realization of longer couplingsections, so that the minimum distance between the microtoroid loopwaveguide and the integrated waveguide can be increased in the couplingregion. This is important, since an excessively small distance betweenthe integrated waveguide and the loop waveguide would lead to anexcessive reduction of the quality factor of the microtoroid due to loopwaveguide scattering losses induced by the presence of the integratedwaveguide.

Phase velocity matching or sufficiently reducing phase velocity mismatchcan be challenging since the core of the microtoroid is made out of thereflow film while the core of the integrated waveguide is made out of amaterial with a higher refractive index. For this reason the effectiveindex of the integrated waveguide is typically higher than the effectiveindex of the loop waveguide. This does not however mean that it isalways the case. First, by locally reducing the dimension of theintegrated waveguide core in the vicinity of the coupling region,typically by reducing the width, that is to say the waveguidecross-section dimension in the direction parallel to the surface of thechip, the overlap of the waveguide mode with the cladding is increasedand the effective index of the waveguide reduced. Furthermore, thewaveguide mode of the integrated waveguide also overlaps with thecladding regions outside of the reflow film. For example, if theseoutside cladding regions are air, or another material with a lowerrefractive index than the reflow film, the effective index of thewaveguide can even be locally reduced below the refractive index of thereflow film, or matched to the refractive index of the reflow film. Theeffective index of the loop waveguide is typically very close to therefractive index of the reflow film since it is a large, typicallymulti-mode waveguide made out of the material of the reflow film and theexcited modes are typically low order modes. In the asymmetricalwaveguide example described above, the effective index of the integratedwaveguide can be matched to the effective index of the loop waveguide ifthe width of the silicon core of the integrated waveguide is locallyreduced (tapered down) to 160 nm. If the distance between the center ofthe loop waveguide to the integrated waveguide is 6.9 μm the couplingcoefficient is such that a 6 μm coupling length is sufficient to obtaincritical coupling assuming an unloaded quality factor of 100e6 for themicrotoroid. Locally reducing the waveguide core cross-section of theintegrated waveguide in the coupling region not only helps to achievephase velocity matching but also increases the modal overlap between theintegrated waveguide and the loop waveguide, so that coupling betweenthe waveguides is also enhanced.

Since the integrated waveguide is typically tapered down to a reducedcross-section in the region where it is coupled with the microtoroid,its mode is also less confined to the waveguide core in this region andhas a higher overlap with the regions outside of the reflow film. Forthis reason, the integrated waveguide is typically at least partiallylocated on the reflow film or within the reflow film in a region wherethe reflow film is undercut. Otherwise the integrated waveguide modewould reach down to the substrate and leak into the substrate. The loopwaveguide mode also needs to substantially reach the integratedwaveguide core in order to couple to the integrated waveguide. This isanother reason why the integrated waveguide is typically at leastpartially located in a region where the reflow film is undercut, sinceotherwise the loop waveguide mode would also reach the substrate andleak into the substrate in the region where it reaches the integratedwaveguide. Both constraints do not apply if the substrate has a lowerrefractive index than the reflow film.

The integrated waveguide is typically also at least partially located ina region where the reflow film is not undercut, since it is typicallyrouted away from the microtoroid over a substantial distance. The widthof the undercut region is typically at most a few 10 s or a few 100 s ofμm, and typically between 1 to 30 μm. Since the regions where thecoupled to integrated waveguide, or other integrated waveguidesoptically connected to the couple to waveguide are not located on orwithin an undercut reflow film are efficiently heat sunk via the chipsubstrate and do not heat up as much during the reflow step, theseregions are preferred for the location of devices with a low thermalbudget such as can be the case for modulators, switches, photodetectors,interleavers, multiplexers or other opto-electronic devices. Since theundercut regions typically heat up substantially during reflow, they aretypically regions where devices with a low thermal budget are notlocated.

Typical dimensions for the toroid outer radius, or for the smallestbending radius of the microtoroid loop waveguide, are between 25 μm and5 mm. In many applications it is beneficial to keep the circumference assmall as possible, to maintain a high free spectral range, a highfinesse, or to highly enhance the optical field intensities within themicrotoroid. In order to maintain a high quality factor, bending radiiare then typically between 50 μm and 1 mm. Typical buried oxide layersand typical reflow films are between 400 nm and 3 μm. Below 400 nm thesubstrate coupling losses are excessively high for the integratedwaveguide, on the order of 100 dB/cm. Above 3 μm, the reflow film is solarge that it becomes very difficult to fabricate microtoroids withsufficiently low radiative losses to maintain a high quality factor.When optimizing the trade-off between integrated waveguide substratelosses and microtoroid bending losses, the resulting optimum buriedoxide layers or reflow films are between 550 nm and 1.5 μm. The innerradius of the microtoroid is typically between 0.8 times to 6 times thereflow film thickness (or the largest vertical dimension of the loopwaveguide is between 1.6 and 12 times the reflow film thickness).

In a preferred embodiment the microtoroids and the coupled to integratedwaveguides are fabricated out of silicon on insulator (SOI) material.The substrate of the apparatus is the bulk silicon (also called siliconhandle or silicon substrate) of the SOI material. The coupled towaveguide is fabricated in the top silicon layer of the SOI materialincluding a tapered region of the waveguide where the waveguide has across-section substantially below the maximum dimension still allowingsingle mode transmission, with the width of the waveguide typicallybelow 50% of the maximum width still allowing single mode transmission.Additional process steps such as etching, chemical mechanical polishing,implantations and material growth can be used to fabricate furtheroptical, electro-optic or electronic devices. Additional back-end layerscan be deposited after fabrication of said waveguide, includingreflowable layers and including metal interconnect layers. A hole isetched into the chip (including processing at the chip or at the waferlevel) corresponding to the precursor for the inverted microtoroid andreaching down into the silicon handle. The silicon of the silicon handleexposed through the hole is etched in such a manner as to undercut theburied oxide for a long enough distance such that the coupled towaveguide is typically at least partially located on an undercut regionof the buried oxide, and so that the waveguide portion that is undercutat least partially corresponds to the tapered down region of the siliconwaveguide. The rim of the precursor hole is then reflown to create themicrotoroid. Optionally, a back-end thinning etch can be selectivelyapplied to the chip in the vicinity of where the microtoroid is locatedin the finished good, either prior to etching the precursor hole, afteretching the precursor hole, or after the silicon substrate etch, butbefore the reflow step. The reflow film then corresponds to the buriedoxide as well as remaining reflowable back-end layers after back-endthinning deposited on top of the buried oxide, such as deposited silicondioxide or deposited silicon oxi-nitride layers.

Typical top silicon SOI layer thicknesses are between 100 nm and 350 nm.In the coupling region, the minimum distance between the integratedwaveguide and the microtoroid is typically between 500 nm to 10 μm,measured as the length between the waveguide and the inner edge of theinverted toroid (where the loop waveguide ends into the remaining hole),minus the largest vertical dimension of the microtoroid (i.e.,essentially equal to the distance between the integrated waveguide andthe center of the loop waveguide minus the inner radius).

In a preferred embodiment the microtoroids and the coupled to integratedwaveguides are fabricated out of silicon on insulator (SOI) material.The substrate of the apparatus is the bulk silicon (also called siliconhandle or silicon substrate) of the SOI material. The coupled towaveguide is fabricated in the back-end of the chip, i.e., in layersdeposited on top of the chip, for example with a waveguide corecontaining silicon nitride or amorphous silicon. Other optical,electro-optic and electronic devices can be fabricated, includingsilicon waveguides with a core defined in the top silicon layer of theSOI material as well as coupling structures that allow coupling lightback and forth between the silicon integrated waveguides and theback-end integrated waveguides. Additional back-end layers can bedeposited after fabrication of said waveguides, including reflowablelayers and including metal interconnect layers. A hole is etched intothe chip corresponding to the precursor for the inverted microtoroid andreaching down into the silicon handle. The silicon of the silicon handleexposed through the hole is etched typically in such a manner as toundercut the buried oxide for a long enough distance such that thecoupled to waveguide is at least partially located on an undercut regionof the buried oxide. The rim of the precursor hole is then reflown tocreate the microtoroid. Optionally, a back-end thinning etch can beselectively applied to the chip in the vicinity of where the microtoroidis located in the finished good, either prior to etching the precursorhole, after etching the precursor hole, or after the silicon substrateetch but before the reflow process. The reflow film then corresponds tothe buried oxide as well as reflowable back-end layers remaining afterback-end thinning deposited on top of the buried oxide, such asdeposited silicon dioxide or deposited silicon oxi-nitride layers.

Similar structures can also be fabricated by starting with an oxidizedsilicon wafer, wherein said oxide at least partially corresponds to thereflow film and wherein additional materials are deposited on top saidoxide out of which the core of the coupled to integrated waveguide isfabricated, including amorphous silicon, silicon oxi-nitride and siliconnitride.

Similar structures can also be fabricated wherein the reflow materialentirely corresponds to back-end layers deposited on top of the chip andwherein the coupled to integrated waveguide is part of the back-endstack of the chip. This is for example compatible with fabricating thesecoupled microtoroids on bulk silicon (non SOI) chips where a substantialportion of the optical devices is also defined in the back-end of thechip.

It is understood that described processing steps can occur at the chipor at the wafer level.

In this invention description, the inverted microtoroid is defined asbeing connected to the non-reflown portion of the reflow film such thatsaid non-reflown portion of the reflow film is located outside of thecircumference of the microtoroid. Reflow film is synonymous withreflowable film. Connected means here that the microtoroid is connectedto a film with essentially the same material or materials as themicrotoroid, the reflow film, with material that is essentially the sameas the microtoroid, without interruption. In other words, the mechanicalconnection occurs all with essentially the same material, since themicrotoroid is made by reflowing the reflow film and is thus directlyconnected to the reflow film. In the case where the reflow film isentirely made out of a unique material such as silicon dioxide, thematerial of the reflow film, of the microtoroid and of the connection isall exactly the same. In the case where the reflow film is composed outof several films, the material of the reflow film, of the microtoroidand of the connection is essentially the same, with the only distinctionbeing that the several films are completely or partially mixed withinthe microtoroid.

The circumference of the microtoroid precursor, for example a hole foran inverted toroid or a disk for a non-inverted toroid, has anunambiguous circumference since these objects are essentially 2Dobjects. In the case of a microtoroid, which is a 3D object, thecircumference is defined as the loop formed by the line where thereflowable film is interrupted at the edge of the microtoroid whenlooking top down onto the top surface of the chip, i.e. for anon-inverted microtoroid it is the furthest the reflowable film extendsfrom the center of the microtoroid or equivalently it is given by theouter edge of the microtoroid and for an inverted microtoroid it is theclosest the reflow film approaches the center of the microtoroid orequivalently it is given by the inner edge of the microtoroid.

The precursor hole for an inverted microtoroid can also be a ring shapedhole if an unetched region is left around the center of the structure,thus forming an isolated island, or more complicated shapes as long asthe microtoroid is formed by thermally reflowing the outmost edge ofsaid hole, since features located inside the hole do not impact thefabrication or functionality of the microtoroid. In this case theinverted microtoroid is not directly connected to the unetched regionsleft over within the circumference of the microtoroid, since they areisolated islands.

The etched regions defining the precursor for a non-inverted toroidtypically take the form of a ring shaped hole, wherein the inner edge ofthe ring shaped hole corresponds to the circumference of the precursorand is the locus where thermal reflow is applied. The outer edge of thehole is not directly connected to the inner edge of the hole and doesnot impact the reflow process or the functionality of the microtoroid.If larger areas are etched during precursor shape definition, the etchedregion can take a different form as long as the microtoroid is formed bythermally reflowing an inner edge of said hole. For example the hole cancover the entire chip with the exception of the microtoroid precursor,or the hole can cover the entire chip with the exception of themicrotoroid precursor and other regions not directly connected to themicrotoroid precursor.

Directly connected refers to continuous connection of two portions ofreflowable film via further portions of reflowable film.

FIG. 1 shows the cross-section of the structure. A microtoroid 1 isformed by reflowing a first film area 20 of reflow film 6 such that thereflow film 6 is located within the circumference of the microtoroid.The film 6 is attached to the rest of the chip via a second film area 22and a pedestal 2. The rest of the reflow film 3 is outside thecircumference of the microtoroid and is not directly connected to themicrotoroid (the gap between the reflow film 3 and the microtoroid 1 canbe air, vacuum, some organic or partially organic material, or someother material different from the material or materials of the reflowfilm). An integrated waveguide 4 is located within the reflow filmconnected to the microtoroid and is trapped within the circumference ofthe microtoroid, since there is no direct connectivity between theportion 6 of the reflow film and the portion 3 of the reflow film. Themicrotoroid is formed by looping a loop waveguide supporting an opticalmode 5. The chip substrate 7 is etched and undercuts the reflow filmwhere it forms the microtoroid. The loop waveguide and the microtoroidhave an inner edge 11 where it is connected to the reflow film and anouter edge 12 where it is not connected to the reflow film. The geometryof the microtoroid can be described by the inner radius 13 and the outerradius 14, the latter when the circumference is circular, or by abending radius 14 for more general shapes.

An inverted microtoroid 10 is connected to the reflow film 6 locatedoutside of the circumference of the microtoroid. It is formed by loopinga loop waveguide that supports an optical mode 5. The microtoroid has aninner edge 11 where it is not connected to the reflow film and an outeredge 12 where it is connected to the reflow film. A coupled tointegrated waveguide 4 is located inside the reflow film outside thecircumference of the microtoroid and is not confined to thecircumference of the microtoroid. The microtoroid geometry can bedescribed by an inner radius 13 and an outer radius 14 when thecircumference is circular, or by a bending radius 14.

The integrated waveguide 4 is coupled to the microtoroid in two couplingregions 15. The integrated waveguide 4 is further electrically contactedwith two metal traces 17 and 18 such that the metal traces and theactive waveguide area connected by the metal traces are in thenon-undercut region of the reflow film 6 so as to minimize thermalexposure during the reflow process. In this example the active waveguidearea can for example be used to induce a phase shift and thus to adjustthe overall coupling strength between the microtoroid and the integratedwaveguides. In general there might be one or several coupling regionsbetween the integrated waveguide and the inverted microtoroid.

A line 19 represents the edge of the reflow film and of the microtoroid.Higher optical field intensities are represented by darker areas withinthe microtoroid and the reflow film. In the case of the thicker reflowfilm structure described above, the low loss mode 5 is not the groundmode as in the case of the thinner reflow structure described above, buta higher order mode that has two lobes in the vertical direction.

An integrated waveguide 4 located within the reflow film 6 is coupled toan inverted microtoroid 10. Even though it is not visible in the chosenscaling of the color map, the microtoroid mode 5 (equivalently the loopwaveguide mode 5) actually reaches to the integrated waveguide core 4 sothat the microtoroid and the integrated waveguide (equivalently the loopwaveguide and the integrated waveguide) are coupled to each other. Thefigure also shows the integrated waveguide mode 24.

The leftmost flow corresponds to the fabrication of an invertedmicrotoroid such that the integrated waveguide 4 made from a siliconlayer 23 is located on the reflow film. The second flow is a flow suchthat the integrated waveguide 4 is located inside the reflow film butoutside of the reflown region, i.e., the integrated waveguide is locatedin the second film area 22 of the reflow film. The third flow is a flowsuch that the integrated waveguide 4 is located inside the reflow filmand inside of the reflown region, i.e. in a first film area 20. Theforth flow is a flow where the back-end is thinned prior to the reflowprocess. These fabrication flows are particularly well suited tofabrication stating with a silicon on insulator (SOI) material, in whichcase the silicon layer 23 is then the top silicon layer of the SOImaterial (sometimes called the device layer in the electronicsliterature).

A silicon waveguide 21 is also shown in order to illustrate that thechip can contain other types of integrated waveguides than the coupledto integrated waveguide 4. FIG. 8 also illustrates that most of thefabrication of the integrated optics and other devices such aselectronic devices can occur before the microtoroid fabrication. Thisfigure can be applied to an inverted or a non-inverted microtoroid.

FIG. 9 shows one example of a hollow core waveguide coupled to themicrotoroid. A silicon waveguide 21 is also shown in order to illustratethat the chip can contain other types of integrated waveguides than thecoupled to integrated waveguide 4. The box shaped waveguide core is ahollow core filled with a material with a lower refractive index thanthe box, such as materials constituting the reflow film. The hollow corecan also be filled with air or vacuum. This figure can be applied to aninverted or a non-inverted microtoroid.

What is claimed is:
 1. A photonic apparatus comprising: an integratedwaveguide, an integrated resonator in the form of a microtoroid and athermally reflowable film, wherein: said reflowable film is defined byone of a thin film and a stack of thin films and comprises a thermallyreflown first film area and a second film area directly connected withthe first film area, said microtoroid is formed in said thermallyreflown first film area, and said second film area is not reflown in theimmediate vicinity of the microtoroid, said microtoroid is opticallycoupled to said integrated waveguide located on or within at least oneof said first or second film areas, and said microtoroid having an edgeextending along a circumference of the microtoroid.
 2. The apparatus ofclaim 1, wherein the integrated waveguide comprises a silicon waveguide,a silicon nitride waveguide, a silicon-oxinitride waveguide, anamorphous silicon waveguide or another waveguide that contains silicon.3. The apparatus of claim 1, wherein the microtoroid and the integratedwaveguide are made on a silicon-on-insulator chip and the reflowablefilm is at least partially constituted out of buried oxide of thesilicon-on-insulator material.
 4. The apparatus of claim 1, wherein saidsecond film area is located outside of the circumference, and saidmicrotoroid is an inverted microtoroid.
 5. The apparatus of claim 1,wherein said second film area is located inside of the circumference,and said microtoroid is a non-inverted microtoroid.
 6. The apparatus ofclaim 1, wherein the microtoroid is at least partially made out of asilicon-on-insulator material and the reflowable film is at leastpartially constituted out of buried oxide of the silicon-on-insulatormaterial.
 7. The apparatus of claim 1, wherein the integrated waveguideis located outside of the circumference of the microtoroid.
 8. A methodof fabricating a photonic apparatus comprising an integrated waveguideand an integrated resonator in the form of a microtoroid coupled to saidintegrated waveguide, said method comprising the following steps:fabricating an integrated waveguide in, or on top of, a reflowable filmor a stack of reflowable films; etching a shape defining a circumferenceinto the film or stack of thin films; etching into material or materialsbelow the reflowable film or stack of reflowable films undercutting thereflowable film or stack of reflowable films at the circumference of theshape by etching into the material or materials below the reflowablefilm or stack of reflowable films; and forming a microtoroid bythermally reflowing the reflowable film or stack of reflowable films atthe circumference of the shape, wherein the step of etching the shapeincludes etching the shape with said circumference close to thewaveguide such that the microtoroid is optically coupled to theintegrated waveguide, wherein the shape comprises a hole, and whereinthe forming step comprises reflowing an outermost edge of the hole tocreate an inverted microtoroid.
 9. The method of claim 8, wherein a coreof the integrated waveguide is fabricated out of silicon, siliconnitride, silicon oxi-nitride, amorphous silicon, or is at leastpartially made out of another material containing silicon.
 10. Themethod of claim 8, wherein: the fabricating step comprises fabricatingthe integrated waveguide at least partially out of one of (i) a topsilicon layer of a silicon-on-insulator material, or (ii) a thin opticalfilm or multiple thin optical films deposited onto asilicon-on-insulator material; and the step of etching the shapeincludes etching the shape into the silicon-on-insulator material so asto reach a silicon handle of the silicon-on-insulator material; themethod further comprises etching the silicon handle so as to undercutburied oxide at the circumference of the shape, and the thermallyreflowing step comprises thermally reflowing the buried oxide and otherlayers located on top of the buried oxide at the circumference of theshape, thereby forming the microtoroid, wherein the buried oxide andother layers located on top of the buried oxide at the circumference ofthe shape at the time the reflowing step is performed comprise thereflowable film or stack of reflowable films, so that the integratedwaveguide is located on or within the stack of partially reflown layers.11. The method of claim 10, wherein: the shape comprises one of a disk,a racetrack, an oval, or another shape with a circumference configuredsuch that the buried oxide remaining in a vicinity of the circumferenceis located inside the circumference, the step of etching the shapeincludes etching the buried oxide and the other layers located on top ofthe buried oxide at the circumference of the shape outside thecircumference of the shape in an immediate vicinity of the circumferenceof the shape, and the formed microtoroid is a non-inverted microtoroid.12. The method of claim 10, wherein: the shape is a hole having anoutmost edge in the shape of a circle, a racetrack, an oval, or anothershape, said outmost edge defines the circumference of the shape suchthat the portion of the buried oxide remaining in a vicinity of thecircumference is located outside the circumference, the step of etchingthe shape includes etching the buried oxide and the other layers locatedon top of the buried oxide at the circumference of the shape inside thecircumference of the shape in an immediate vicinity of the circumferenceof the shape, and the formed microtoroid is an inverted microtoroid. 13.The method of claim 11, further comprising the step of partiallyremoving the film or stack of films located on top of the buried oxidein the vicinity of the circumference of the shape, and thereby thinningthe film or stack of films, prior to the thermally reflowing step and(i) prior to etching the shape, (ii) after etching the shape but priorto undercutting the buried oxide film, or (iii) after undercutting theburied oxide film but prior to thermally reflowing the buried oxide filmand layers remaining on top of the buried oxide film in the vicinity ofthe circumference.
 14. The method of claim 12, further comprising thestep of partially removing the stack of films located on top of theburied oxide in the vicinity of the circumference of the shape andthinning the stack of films prior to the thermally reflowing step,wherein said thinning can take place prior to etching the shape, afteretching the shape but prior to undercutting the buried oxide film, orafter undercutting the buried oxide film but prior to thermallyreflowing the buried oxide film and layers remaining on top of theburied oxide film in the vicinity of the circumference.
 15. Theapparatus of claim 4, wherein said microtoroid is not directly connectedto further portions of the reflowable film located inside thecircumference.
 16. The apparatus of claim 4, wherein said circumferencedefines a hole covering an entire area within the circumference ordefines the outmost edge of a hole having a more complex shape.
 17. Theapparatus of claim 5, wherein said microtoroid is not directly connectedto further portions of the reflowable film located outside of thecircumference.
 18. The apparatus of claim 5, wherein said circumferencedefines an inner edge of a ring shaped hole or an inner edge of adifferently shaped hole.